【正文】 greater friction between the bar and the surrounding concrete. However, these pressures would then relieved by the subsequent cracking of the concrete, which would contribute to the decrease in the bond strength as crack 19 widths increase. A possible hypothesis is that due to the level of cover, three times bar diameter, the effect of confinement by the stirrups is reduced, such that it has little impact on the bond stress in uncracked concrete. However, once cracking has taken place the confinement does have a beneficial effect on the bond. It may also be that the pressive strength of the concrete bined with the cover will have an effect on the bond stresses for uncorroded specimens. The data presented here has a cover of three times bar diameter and a strength of 40 MPa, other research ranges from to four times cover with pressive strengths from 40 to 77 MPa. Comparison of 12 and 16 mm rebar The maximum bond stress for 16 mm unconfined bars was measured at MPa and for the 12 mm bars it was MPa. These both corresponded to the control specimens with no corrosion. The unconfined specimens for both the 12 and 16 mm bars showed no increase in bond stress due to corrosion. For the confined specimens the maximum bond stress for the control specimens were MPa for the 12 mm bars and MPa for the 16 mm bars. The maximum bond stress for both sets of confined specimens corresponded to point of the initial cracking. The maximum bond stresses were observed at a mean crack width of mm for the 12 mm bars and mm for the 16 mm bars. The corresponding bond stresses were, and MPa. Overall the 12 mm bars displayed higher bond stresses pared to the 16 mm bars at all crack widths. This is attributed to a different failure mode. The 16 mm specimens demonstrate splitting failure while the 12 mm bars bond failure. Effect of casting position 20 There was no significant difference of bond strength due to the position of the bar (top or bottom cast) once cracking was observed, Fig. 15. For control specimens, with no corrosion, however, the bottom cast bars had a slightly higher bond stress than the top cast bars. These observations are in agreement with other authors [4, 11, 15, 22]. It is generally accepted that uncorroded bottom cast bars have significantly improved bond pared to top cast bars due to the corrosion products filling the voids that are often present under top cast bars as the corrosion progresses [14]. The corrosion also acts as an ‘a(chǎn)nchor’, similar to the ribs on deformed bars, to increase the bond. Overall, the mean value of bond stress for all bars (corroded and uncorroded) located in the top were within 1% of the mean bond stress of all bars located in the bottom of the section—for both unconfined and confined bars. This is probably due to the level of cover. The results reported previously are on specimens with one times cover [14]. However, at three times cover it would be anticipated that greater paction would be achieved around the top cast bars. Thus the area of voids would be reduced and thus the effect of the corrosion product filling these voids and increasing the bond strength would be reduced. Fig. 15 Bond stress versus mean crack width for 12 mm bars, top and bottom cast positions, confined specimen 21 4 Conclusions A relationship was observed between crack width and bond stress. The correlation was better for maximum crack width and bond stress than for mean crack width and bond stress. Confined bars displayed a higher bond stress at the point of initial cracking than where no corrosion had occurred. As crack width increase the bond stress reduced significantly. Unconfined bars displayed a decrease in bond stress at initial cracking, followed by a further decrease as cracking increased. Top cast bars displayed a higher bond stress in specimens with no corrosion. Once cracking had occurred no variation between top and bottom cast bars was observed. The 12 mm bars displayed higher bond stress values than 16 mm with no corrosion, control specimens, and at similar crack widths. A good correlation was observed between bond stress and degree of corrosion was observed at low levels of corrosion (less than 5%). However, at higher levels of corrosion no correlation was discerned. Overall the results indicated a potential relationship between the maximum crack width and the bond. Results shown herein should be interpreted with caution as this variation may be not only due to variations between accelerated corrosion and natural corrosion but also due to the plexity of the cracking mechanism in reality. 22 中文譯文： 約束和無約束的鋼筋對裂縫寬度的影響 收稿日期： 2020 年 1 月 14 納稿日期： 2020 年 12 月 14 日 線上發(fā)表時間： 2020 年 1 月 23 日 摘 要 本報告公布了局限約束和自由的變形對粘結(jié)強度 1 16 毫米鋼筋的表面腐蝕程度和裂紋影響的比較結(jié)果。 同時 還 發(fā)現(xiàn)在 圍箍筋 處發(fā)現(xiàn) 表面裂紋 的地方 粘結(jié)強度增加 ， 而無側(cè)限的 樣本 中 沒有 觀察 到 粘結(jié)強度增加 。這是由于鋼表面形成了腐蝕產(chǎn)物，從而影響了鋼和混凝土之間的粘結(jié)。這種開裂可導(dǎo)致更嚴重的惡化和進一步的 腐蝕。 以往的研究調(diào)查腐蝕對粘結(jié)的影響 [25， 7， 12， 20， 2325， 27， 29]，提出了數(shù)據(jù)模型 [4， 6， 9， 10， 18， 19 24， 29]。 加強鋼筋的腐蝕導(dǎo)致生成鐵氧化物，它的體積大于原鋼材。然而，以混凝土的剝離可以在一定程度上抵消粘結(jié)力的損失。這種撤去偏心或“梁端”模式 樣本 以一個典型的簡支梁錨固區(qū)的粘結(jié)長度支撐。重復(fù)測試有側(cè)限和自由 樣本 。 材料 配合比設(shè)計，如表 1 所示。測 試前水浴養(yǎng)護 28 天。 39。這些相關(guān)實驗使用外加電流或干濕周期人工風(fēng)化和升高溫度延緩腐蝕時間，同時保持惡化機制處于自然狀態(tài)。然而，應(yīng)謹慎應(yīng)用外加電流的加速腐蝕，加速過程并不完全復(fù)制在實際結(jié)構(gòu)中所涉及的機制。金屬板和混凝土之間放置海綿（用鹽水噴灑）提供足夠的接觸，如圖 2。表面裂紋寬度沿鋼筋長度測量間隔 20mm，從約束（塑料管）末端開始 20mm 用斷路器光學(xué)顯微鏡測量。 粘結(jié)強度測試通過手動操作液壓千斤頂和一個定制的試驗裝置，如圖 3所示。給 樣本 足夠剛 性的約束可以確保在加載過程中最小的旋轉(zhuǎn)或扭曲。這是由于腐蝕和開裂是一個動態(tài)的過程，裂縫 是以 不同的速度傳播的。初始裂縫發(fā)生在很短的時間內(nèi)，通常在幾天之內(nèi)產(chǎn)生。一般情況下無側(cè)限的 樣本 只有僅有的一部分裂縫，而自由的 樣本 裂縫的發(fā)展卻十分常見，觀察到的裂縫垂直對齊下邊，垂直向下側(cè)相鄰的鏈接，如圖 5。 圖 6拉出后縱向開裂 圖 7角開裂后拉出 31 鋼筋最初（預(yù)制）由 12％的鹽酸溶液清洗，然后在蒸餾水清洗，另外蒸餾水洗滌之前由氫氧化鈣溶液中和。多數(shù)表現(xiàn)出可見的凹陷，類似的實際結(jié)構(gòu)，如圖 9。圖 12 和圖 13 顯示的最大裂縫寬度的數(shù)據(jù)。 16 毫米的 樣本 增加約 14％。粘結(jié)強度，腐蝕 程度，鋼筋尺寸，保護層，節(jié)點的詳細信息和拉伸強度之間的異變由羅德里格斯預(yù)測等已經(jīng)被詳細討論。實驗結(jié)果（ 14 和 25％以上）是這些值的 6070％